The piston is one of the most important but vulnerable components in various kinds of internal combustion engines. As the engine is pushed to higher and higher thermal efficiency, the piston is required to work in a more hostile operating environment. One of the working limits for a piston is the maximum temperature that a piston can sustain. This is especially important for aluminum-alloy based pistons, which have a pronounced temperature dependence on their mechanical properties. The rapid fall-off of the mechanical properties of the piston alloy at temperatures above 200.degree. C. is responsible for piston ring sticking and piston material transfer due to contact wearing. Aluminum-alloy based pistons also have a very large coefficient of thermal expansion. Problems occurring as a result of piston temperature increase also include coking, deterioration of lubricants, and increase in the designed clearance between the piston and cylinder liner, which can result in noise and vibration due to piston slapping. All of these problems would lead to a dramatic decrease in engine thermal efficiency and service life. For diesel engines, a higher engine thermal efficiency and less smoke due to improvement in air utilization could be realized by increasing the engine compression ratio. This will in turn result in a higher engine working temperature and a higher piston temperature in the ring groove region of the piston.
Piston cooling is a critical measure to achieve a higher engine performance. However, it is difficult to implement the cooling due to the reciprocal motion of the piston. Transferring heat away from the piston through the cylinder wall is also limited due to the small contact area between the piston and the cylinder wall. A commonly used method for cooling pistons is the crankcase oil splash/mist undercrown cooling. Above a certain rating, additional oil cooling is necessary and this is required for medium and high speed engine pistons. Provision of an internal cooling gallery allows a larger cooling surface and a shorter heat flow path as compared to the undercrown cooling. However, the gallery has a reputation of causing stress concentration and reducing piston strength in the ring groove region. Also, since the gallery is located in the upper section of the piston, it is difficult for the cooling oil to reach the gallery from the crankcase. Jet cooling has also been used to cool the upper section of the piston. The oil may be supplied from a standing jet fixed in the crankcase or through a drilled connecting rod. Although jet cooling may be more effective than splash/mist cooling, accurate jet alignment and capture efficiency are practically not without problems due to the rapid reciprocating motion of the piston.